Solar magnetic fields from chromospheric emissions to the large-scale dipole

The Sun not only sustains life on Earth but is an active star that continuously shapes conditions in its atmosphere, near-Earth space, and even on the Earth’s surface. This activity is driven by a magnetic field generated deep inside the Sun, which emerges at the surface as strong magnetic concentrations known as active regions. This thesis investigates how the magnetic field of these regions at the solar surface influences the Sun’s atmospheric layer above the surface, the chromosphere, and how individual active regions collectively shape the Sun’s global magnetic field.

Active regions often appear as bright structures in chromospheric emissions. Previous studies have shown that the magnetic field strength of active regions determined from chromospheric emissions is approximately constant. This work shows that the magnetic field strength of active regions that were determined from satellite observations of the chromosphere is nearly twice as high as earlier low-resolution estimates. This research recognizes that this difference primarily stems from the complex boundary areas of active regions, which in high-resolution satellite data can be better distinguished from the weak background field. The thesis also shows that the radiative and magnetic properties of the active region boundary differ significantly from the inner parts of active regions. A major new finding is that the intensity of chromospheric radiation depends not only on the magnetic field strength but also on its orientation relative to the solar surface.

The magnetic field of active regions spreads across the solar surface, forming the global magnetic field. This field partially escapes into space and surrounds Earth and other planets, connecting their magnetic fields to the Sun’s. In the thesis a new method was developed for determining the strength and orientation of the global magnetic field. The method produces results consistent with more complex coronal models and connects the amount of magnetic flux escaping from the Sun to the Sun’s dipole field. This new approach provided the means to analyze the development of active regions and their contribution to the Sun’s global field much more effectively than before. The results particularly highlight the importance of the longitudinal distribution of active regions.

Tools developed in the thesis provide new means to understand how the Sun’s global magnetic field evolves under the influence of active regions. The results can, for example, help explain why geomagnetic activity observed on Earth continued to increase even as solar activity declined in the latter half of the 20th century. Furthermore, the findings may potentially be used to predict solar activity on a timescale of about six months, which is relevant, for instance, for forecasting average winter temperatures in Finland.